1. Field of the Invention
The present invention relates to a method for controlling a tilt servo in an optical disc device, and more particularly to an optical disc device and a method for correcting a tilt of an optical disc using a focus error signal and a focus drive signal.
2. Description of the Related Art
Recently, optical discs capable of storing digital video and audio data, for example, digital versatile discs (DVDs) or compact discs (CDs), are widely supplied. Moreover, optical disc devices equipped with an optical pickup for reproducing or recording data on optical discs have been commercialized.
An optical pickup provided in an optical disc device comprises an objective lens 11, a beam splitter 12, a photo detector (PD) 13, a laser diode (LD), as shown in FIG. 1.
In a process for manufacturing the optical disc device, a base on which the optical pickup is mounted is finely adjusted so that the objective lens 11 of the optical pickup is parallel with a skew-free optical disc 10 loaded into the optical disc device.
This is to prevent playback or recording performance from being degraded when the objective lens 11 and the optical disc 10 are not parallel to each other. Thus, the base is finely adjusted in tangential and radial directions on the basis of the skew-free optical disc 10. This fine adjustment is referred to as the tilt or skew adjustment.
As shown in FIG. 2, the optical disc device of FIG. 1 also comprises an LD driver 15, a focus error (F.E.) detector 16, a microcomputer (Micom) 17, a focus drive 18, a servo 19, a memory 20, etc.
The F.E. detector 16 carries out a mathematical operation on electrical signals outputted from the photo detector 13 included in the optical pickup, for example, a 4-element photo detector divided into A, B, C and D areas, and then detects a focus error signal (F.E.=(A+C)−(B+D)).
In order to bring the value of the focus error signal down to zero, the microcomputer 17 generates and outputs a focus control signal based on the focus error signal. The servo 19 generates a focus drive signal corresponding to the focus control signal and then outputs the generated focus drive signal to the focus drive 18.
The focus drive 18 applies, to a focus actuator coil (not shown) of the optical pickup, focus drive current corresponding to the focus drive signal. Through this operation, a focus servo operation is performed to vertically shift the objective lens 11 so that a detected focus error can be reduced.
Meanwhile, where a skew is present in the optical disc inserted into the optical disc device, for example, where the optical disc is bent in the outward direction (or the skew is present in the radial direction), the optical disc device operates in a state in which the optical disc and the objective lens of the optical pickup are not positioned parallel to each other while being at a tilt. Thus, the performance of the focus servo or a tracking servo is degraded and hence the performance of a playback or recording operation is degraded. This phenomenon is called a tilt error.
In order for the tilt error to be corrected, jitter characteristics of a radio frequency (RF) outputted from a photo detector (PD) must be measured to calculate a tilt degree between the optical disc and the objective lens of the optical pickup. Alternatively, the tilt degree must be measured by a special tilt sensor. That is, an expensive jitter measurement circuit or a tilt sensor occupying a large space is required and used to correct the skew of the optical disc. However, this increases the size and price of the optical disc device, which is a problem.